Is it procrastination?
If you're consciously deciding to give the e-mail some thinking time (rather than just putting it off - explained below), I don't think so.
Putting something on the back burner while you "let the idea stew" is a legitimate approach to creativity and decision-making - as long as you give it a bit of active thinking before it does go on the back burner. It's a psychological process called incubation, with lots of research behind it (more info on this relevant Stack Exchange CogSci question, "How is it that taking a break from a problem sometimes allows you to figure out the answer?").
Incubation is defined as a process of unconscious recombination of thought elements that were stimulated through conscious work at one point in time, resulting in novel ideas at some later point in time.
How to stop it turning into procrastination
Of course, it's important not to use incubation as an excuse for all procrastination. If you've consciously made a decision to spend time thinking about something rather than doing it now, spend 10 minutes making some initial notes to get your brain working on the idea. Put an item in your diary (or scheduled in your to-do system) for, say, a couple of days' time to finish the task and pull the trigger (hit send, make the phone call, whatever). By this point you'll probably find you've figured out a lot of a awkwardness of a difficult e-mail, and you can write it much more comfortably, confident that you're not being hasty.
Haven't made your initial notes? Haven't put a date to finish the task in your calendar? That's not incubating, that's procrastinating :-)
I've always liked venture capitalist Fred Wilson's related tip about starting a big project as soon as you can, if only for one hour, so that all the rest of the time you're "getting around to it" your brain is incubating the ideas.
Applying this technique to purchase decisions
You mention your question also applies to decisions like whether to spend money on something - as it happens, I use a similar technique to stop impulse purchases. Fancy buying something, but unsure whether you should? Put it in your calendar for a month's time - if you still need/want it (and can afford it), you can buy it guilt-free, knowing it's not an impulse purchase, and is something that you do actually want, as you've thought about it over a period of time. Nine times out of ten, I find I don't want/need whatever it was any more.